Mechanical Engineering Department, Ferdowsi University of Mashhad, Iran.
Ann Biomed Eng. 2010 Sep;38(9):2817-28. doi: 10.1007/s10439-010-0043-1. Epub 2010 Apr 29.
The purpose of this study is to investigate the effects of the Reynolds number and the bend sweep angle on the blood flow patterns of S-shaped bends. The numerical simulations of steady flows in S-shaped bends with sweep angles of 45 degrees , 90 degrees , and 135 degrees are performed at Reynolds numbers of 125, 500, and 960. Hemodynamic characteristics such as secondary flows, vorticity, and axial velocity profiles are analyzed in detail. Flow patterns in S-shaped bends are strongly dependent on both Reynolds number and bend sweep angle, which can be categorized into three groups based on the first bend secondary flow effects on the transverse flow of the second bend. For low Reynolds numbers and any sweep angles, secondary flows in the second bend eliminate the first bend effects in the early sections of the second bend and therefore the axial velocity profile is consistent with the bend curvature, while for high Reynolds numbers depending on the bend sweep angles the secondary vortex pattern of the first bend may persist partially or totally throughout the second bend leading to a four-vortex secondary structure. Moreover, an interesting flow feature observed at the Reynolds number of 960 is that the secondary flow asymmetrical behavior occurred around the second bend exit and along the outflow straight section. This symmetry-breaking phenomenon which has not been reported in the previous studies is shown to be more pronounced in the 90 degrees S-shaped bend as compared to other models considered here. The probability of flow separation as one of the important flow features contributing to the onset and development of arterial wall diseases is also studied. It is observed that the second bend outer wall of gentle bends with sweep angles from 20 degrees to 30 degrees at high enough Reynolds numbers are prone to flow separation.
本研究旨在探讨雷诺数和弯道扫掠角对 S 形弯道血流模式的影响。对雷诺数为 125、500 和 960 时扫掠角分别为 45 度、90 度和 135 度的 S 形弯道进行了稳态流动的数值模拟。详细分析了二次流、涡度和轴向速度分布等血液动力学特性。S 形弯道中的流动模式强烈依赖于雷诺数和弯道扫掠角，可以根据第一弯道对第二弯道横向流动的二次流效应将其分为三组。对于低雷诺数和任意扫掠角，第二弯道中的二次流会消除第二弯道早期部分的第一弯道效应，因此轴向速度分布与弯道曲率一致；而对于高雷诺数，则取决于弯道扫掠角，第一弯道的二次涡旋模式可能会部分或完全持续到第二弯道，从而导致四涡二次结构。此外，在雷诺数为 960 时观察到一个有趣的流动特征，即二次流在第二弯道出口处和沿流出直段出现非对称行为。这种在之前的研究中没有报道过的打破对称的现象在 90 度 S 形弯道中比在其他考虑的模型中更为明显。作为导致动脉壁疾病发生和发展的重要流动特征之一的流动分离的可能性也进行了研究。观察到在足够高的雷诺数下，扫掠角从 20 度到 30 度的平缓弯道的第二弯道外壁容易发生流动分离。
